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 GMC-300 minimal Voltage / measuring HV		  New Topic  Reply to Topic
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hdripper

Switzerland
3 Posts
Posted - 04/20/2015 :  09:30:32  Show Profile  Reply with Quote
Hello everyone

What is typically the lowest voltage that the original GMC-300 has by adjusting the blue pot?


The reason I'm asking is because I wanted to measure the actual tube voltage with a 1 Gigaohm resistor and voltmeter with 10 MOhm input impedance. As described in the sticky thread, this gives a calculated factor of 101.

Now the voltage readings I am getting seem to be too high!

* GMC-300 minimal voltage (pot fully CW) = 5.35V * 101 = 540V (!)

In order to have a comparison to see if these readings make any sense, I have made some tests with another tube and different circuit which can be adjusted lower:

* Expected tube starting voltage: 300V
* Measured tube starting voltage: 445V (!)

* Typical operating voltage: around 390V
* Determined operating voltage: around 550V(!)

So again, the readings where much higher than expected. I then measured the actual input resistance of the voltmeter (9.88 MOhm) and determined the actual resistor value by voltage drop calculations (1088 MOhm) -> This would factor of 110 which is even worse!

Does anyone have a clue on what might be going on?

Cheers
Marco
Reply #1

ZLM

1261 Posts
Posted - 04/21/2015 :  10:17:33  Show Profile  Reply with Quote
Your calculation seems right and tube voltage circuit need to be adjusted.

Check the blue pot, it should have about 50K ohm value. If not, then you may need to confirm the pot if it is in working condition.

Another simple way is to remove the C11 capacitor(it connected to the tube resistor). This should let the tube voltage drop about 30%.
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Reply #2

beckettanne

USA
2 Posts
Posted - 04/22/2015 :  14:29:40  Show Profile  Reply with Quote
I just received my g/c and am trying to figure out how to register it. When I turn on the GQ software (which I had to download as my disc was blank), I am unable to register. Am I not entering the SN correctly? I've tried using the email address (which I used to register, here) and I've tried the user-name (same as here). Neither seems to work in combination with the SN.
Can anyone help me with this, please?
Thank you.
@
annebeckett
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Reply #3

ZLM

1261 Posts
Posted - 04/22/2015 :  23:03:32  Show Profile  Reply with Quote
The Data Logger PRO is not free for all features, you needs to have a registration code in order to enable all features.

The Data Viewer is free software and it does most of features too.
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